This invention relates to a semiconductor laser excitation solid laser and in particular to increasing the efficiency of the solid laser.
A semiconductor laser excitation solid laser is available, for example, as shown in JP-A-5-90672. FIGS. 5 and 6 show the configuration of the semiconductor laser excitation solid laser shown in the gazette. Numeral 61 denotes a semiconductor laser, numeral 62 denotes a collimating lens, numeral 63 denotes a focusing lens, numeral 64 denotes a solid laser medium, numeral 65 denotes a mirror, numeral 66 denotes a beam splitter placed between the collimating lens 62 and the focusing lens 63, numeral 67 denotes a reflection-type grating, and numeral 68 denotes a two-channel photodiode. Numerals 71 and 72 denote amplifiers for amplifying output of the two-channel photodiode, numeral 73 denotes a comparator for making a comparison between output magnitudes of the amplifiers, numeral 75 denotes a temperature adjustment circuit, and numeral 76 denotes a Peltier element for controlling the temperature of the semiconductor laser 61.
Next, the operation is as follows: Apart of light emitted from the semiconductor laser 61 for exciting the solid laser medium 64 is guided into the reflection-type grating 67 through the beam splitter 66. The semiconductor laser light spread by the reflection-type grating 67 is detected by the two-channel photodiode 68. The two-channel photodiode 68 comprises two adjacent diodes placed so that the interfacial boundary therebetween becomes the center wavelength of an absorption spectrum of the solid laser medium 64.
If the barycenter of a radiation spectrum of the semiconductor laser 61 shifts from the center of the absorption spectrum, one of two photodiode outputs of the two-channel photodiode exceeds the other. The output difference is compared by the comparator 73 and the current supplied through the temperature adjustment circuit 75 to the Peltier element 76 installed in the semiconductor laser 61 is controlled.
By the way, the excitation wavelength of the solid laser medium 64 has upper and lower limits. For example, the absorption spectrum corresponding to excitation of Nd:YAG is limited in the vicinity of the range of 790 nm to 820 nm as shown in FIG. 7 and particularly shows a complicated structure indicating sharp absorption in 808 nm and having a comparatively low peak in the vicinity of 805 nm.
On the other hand, the semiconductor laser 61 has a feature that its radiation wavelength shifts depending on the temperature of the active layer of the semiconductor laser 61. An AlGaAs/GaAs-family semiconductor laser having a radiation wavelength in the vicinity of 808 nm contains temperature dependency of about 0.2 to 0.3 nm/xc2x0 C.; the higher the temperature, the longer wavelength side the radiation wavelength shifting. The radiation spectral width is 2 to 3 nm. Therefore, the configuration as described above is constructed, whereby feedback control with the target wavelength as the center can be performed for the radiation wavelength of the semiconductor laser 61.
However, the absorption spectrum of the solid laser medium is complicated and the radiation spectrum of a semiconductor laser is too narrow to cover all the absorption spectrum of the solid laser medium, thus sufficient excitation efficiency cannot be provided simply by controlling the radiation wavelength of the semiconductor laser based only on the center wavelength or the peak wavelength of the absorption spectrum; this is a problem.
With a laser for generating high power or a laser requiring high-quality beam, a plurality of semiconductor lasers are used for excitation. This is because one semiconductor laser cannot provide sufficient excitation power and one solid laser medium needs to be excited uniformly from multiple directions.
To use a plurality of semiconductor lasers, variations in characteristics from one semiconductor laser to another introduce a problem. The characteristics of temperature dependency of radiation wavelength spectrum and wavelength vary from one semiconductor laser to another. The characteristics change with time. Therefore, to perform temperature control of wavelength for each semiconductor laser, a spectroscope and a temperature controller become necessary for each semiconductor laser and inevitably the configuration becomes complicated; this is a problem.
It is an object of the invention to provide a semiconductor laser excitation solid laser for solving the above-described problems and increasing the excitation efficiency of the semiconductor laser excitation solid laser according to a simple configuration.
According to the invention, there is provided a semiconductor laser excitation solid laser comprising a solid laser medium, a semiconductor laser for exciting the solid laser medium, a spectrometer for detecting a wavelength region of a radiation spectrum of the semiconductor laser for exciting the solid laser medium, computation means for normalizing an area of the detected spectrum detected by the spectrometer and computing an overlap area between the normalized detected spectrum and a normalized absorption spectrum involved in laser excitation of the solid laser medium, and temperature control means for controlling the temperature of the semiconductor laser based on the output from the computation means.
Therefore, control based on the areas of the wavelength regions of the absorption spectrum of the solid laser medium and the detected spectrum from the semiconductor laser rather than control based only on the center wavelength or the peak wavelength of a spectrum is performed, whereby a semiconductor laser excitation solid laser of high excitation efficiency making the most of any other absorption spectrum region than the center wavelength or the peak wavelength of the spectrum can be provided.
According to the invention, the semiconductor laser excitation solid laser wherein the temperature of the semiconductor laser is controlled so that the overlap area between the detected spectrum and the absorption spectrum reaches the maximum is provided.
Therefore, optimum control for the characteristics of both the solid laser medium and the semiconductor laser can be performed, and the excitation efficiency of the semiconductor laser excitation solid laser can be furthermore increased.
According to the invention, the semiconductor laser excitation solid laser having a plurality of semiconductor lasers, wherein the whole spectrum provided by combining the spectra shown by the semiconductor lasers is detected as the radiation spectrum to be detected is provided.
Therefore, even with the semiconductor laser excitation solid laser comprising a plurality of semiconductor lasers, the whole spectrum provided by combining the spectra shown by the semiconductor lasers is normalized and computed, so that control with good accuracy can be performed and the excitation efficiency of the semiconductor laser excitation solid laser can be increased.